The present invention is directed to a method of hardening a solid metal object consisting essentially of lead, such as a bullet, and a hardened swaged bullet.
Traditionally bullets have been formed by the casting of molten lead or lead alloys in molds.
In commercial production, when the solidified cast bullet is released from the mold it is typically cooled at ambient temperature in air. As a result, the cast lead or lead alloy bullets rapidly lose any hardness properties which they may have had upon release from the mold during the cooling. Thus, by the time they are to be fired they are quite soft. Unless these cast bullets are jacketed with a harder metal such as copper, this softness limits the muzzle velocity at which the softened cast bullets may be fired if barrel leading is to be maintained at acceptable levels and accuracy is to be maximized.
This softness problem has been alleviated to some extent by commercial producers of cast bullets by the use of so-called "6-2" lead alloys in which the alloy is an alloy of 6 wt % antimony and 2 wt % tin with the remainder being lead. The relatively high amount of antimony in the alloy does impart some increased hardness to the commercially cast bullets. However, these somewhat increased hardness levels are still insufficient for firing at higher muzzle velocities unless the bullet is jacketed.
Another disadvantage in the prior commercial casting procedures is that the production rate is substantially limited. At least one reason for this is that the bullets must be held in the mold until their temperature is below the slump temperature to insure that their molded shape does not change following release from the mold. Thus, the maximum current commercial production rate of cast bullets is only about 3500 bullets per hour from an eight mold, two cavity per mold machine.
In order to achieve higher muzzle velocities in cast bullets without resorting to expensive jacketing, handloaders occasionally have resorted to the heat hardening of the cast bullets, either bullets which the handloader has personally cast or bullets which have been obtained commercially. This hardening has been accomplished either by dropping the just cast hot bullet into water to quench it, or by reheating a previously cast bullet to just below its slump temperature and then quenching it. This tends to freeze the molecular structure and alignment at the heated alignment of the molecules in which the hardness is greater.
The handloader casting and/or hardening procedures also have disadvantages. One obvious disadvantage is that the production rate is substantially less than the commercial procedures which are already relatively low. Where quenching is to be done of bullets dropped directly from the casting mold, precise timing and close temperature control are required to avoid significant variations in hardness which could result in hardnesses of only a fraction of the maximum possible hardness. Moreover, the presence of quenching water in close proximity to the molten lead in the melting ladle or pot is dangerous because if even a few drops of water accidently contact the molten lead, the lead may explode from the ladle.
Cast bullets, whether commercially or handloader produced, also suffer several additional disadvantages. One such disadvantage is the fact that the cast bullet has a seam from the molding equipment. Such seams reduce the aerodynamic qualities of the bullet and, therefore, reduce the bullet's accuracy if it is to be fired in an unjacketed condition.
Another disadvantage of the casting method is that an alloy is typically used which contains a considerable quantity of tin as previously mentioned. Tin is added to enhance the flowability of the molten alloy in the mold, and if tin is not included, the resulting molded product is usually inferior. However, the tin tends to reduce the hardness of the product and the effectiveness of the antimony which has been included for that purpose. Thus, the level of antimony must be increased to compensate for the loss of hardness. However, both the antimony and the tin substantially increase the bullet cost because they are metals which are considerably more expensive than lead.
In order to overcome at least some of the disadvantages inherent in the casting methods and products formed thereby, stamping or swaging procedures for the manufacture of bullets came into existence around the turn of the century. In the swaging procedure a drawn lead or lead alloy wire is cross-sectioned to form a number of small blanks. One of these blanks is then placed in a swaging die which has a cavity of the shape of the finished bullet. The blank is then punched with a ram punch under substantial pressure so that it cold flows in the cavity to assume the shape of the bullet. The finished, shaped wrought swaged bullet is then removed from the forming die. No further processing of the bullet other than preparing it for lubrication and lubricating it is necessary if the bullet is to be used at low muzzle velocities. Where the swaged wrought bullets are to be used at higher muzzle velocities, the swaged bullets typically have been jacketed or plated with copper or the like to increase their outer hardness.
Swage forming of bullets offers several distinct advantages over the casting of bullets. One advantage is that the swaged wrought bullet is seamless. Another distinct advantage is that the swaging process is capable of production rates which greatly exceed those of casting. In swaging up to as many as about 20,000 bullets per hour can be commercially produced from a single die cavity. Still another advantage is that tin which is needed for flowability of the molten lead alloy in the casting process can be eliminated in the swage forming process because flowability is not a concern. Thus, the increased expense and potential reduction in hardness which might be otherwise experienced with the addition of tin is eliminated in swaged bullets, and the levels of antimony may also be reduced.
In a jacketed bullet hardness of the lead is not of particular concern from the standpoint of leading because the jacketing, for example copper, effectively defines the surface hardness of the bullet during firing. However, in an unjacketed bullet in which the lead or lead alloy is in direct contact with the rifling in the barrel of the firearm, hardness is a concern. The lower the hardness, the greater the amount of leading that is deposited in the lands and grooves of the firearm rifling. Increased leading reduces the accuracy. Moreover, as the muzzle velocity of the ammunition increases, the leading also increases.
As previously mentioned, leading is a function of the muzzle velocity of the bullet. The United States Practical Shooting Association has established regulations for competition matches which are based on power factor. Those regulations define the power factor by the formula: ##EQU1## where PF=power factor, W=bullet weight in grains (gr), and V=muzzle velocity in feet per second (fps). For major power factor competition events those regulations currently set a minimum for the major power factor of 175. The ammunition of one who competes under those regulations must equal or exceed that major power factor.
Leading of an unjacketed bullet typically occurs when the muzzle velocity is about 900 fps or more. Typical handgun bullet weights are in the range of about 95-230 gr, and weights of about 115 gr are currently popular because the lighter the bullet, the less the recoil. Thus, it will be seen that where the weight of the bullet is the heavier 230 gr, leading is not a major concern because a muzzle velocity of only about 760 fps is needed to meet the 175 major power factor requirement. However, the muzzle velocity of a 95 gr bullet would be about 1850 fps, and of the currently popular 115 gr bullet would be about 1525 fps. These muzzle velocities will result in excessive leading with a typical unjacketed swage wrought lead bullet.
In the present invention a process for the hardening of a lead or lead alloy swaged wrought solid object, such as a bullet, and a hardened swaged wrought bullet are disclosed in which all of the advantages of a swaged bullet are realized, in which jacketing with its increased cost may be eliminated, but in which lighter bullets may be fired at high muzzle velocities which satisfy the foregoing major power factor requirements without unacceptable leading of the rifling of the firearm and loss of accuracy. Moreover, the tendency of jacketed bullets to increase barrel erosion and shorten the life of the barrel is substantially reduced because the need for jacketing is eliminated.
In one principal aspect of the present invention, a method of hardening a solid metal object which has been swaged cold wrought formed under pressure and in which the metal consists essentially of lead includes heating the swaged cold wrought solid metal object to a temperature near but less than the slump temperature of the metal, and quenching the heated object in a liquid to rapidly reduce its temperature.
In another principal aspect of the present invention, a method of making a hardened swaged wrought bullet includes heating a swaged wrought bullet which has been swage formed under high pressure in a swage forming die to a temperature near but less than the slump temperature of the metal, and quenching the heated wrought bullet in a liquid to rapidly reduce its temperature.
In still another principal aspect of the present invention, the metal is primarily lead, and the swaged wrought object or bullet is heated to about 450.degree. F. for at least about 30 minutes.
In still another principal aspect of the present invention, the quenching liquid is water.
In still another principal aspect of the present invention, a bullet comprises an unjacketed swaged wrought metal bullet having a hardness of at least about 15 Brinell.
In still another principal aspect of the present invention, the metal is lead or an alloy of lead and antimony.
In still another principal aspect of the present invention, the amount of antimony in the metal alloy does not exceed about 5 wt %, and preferably about 3.5 wt % of the total weight of the metal.
In still another principal aspect of the present invention, the metal is substantially free of tin.
In still another principal aspect of the present invention, the bullet is seamless.
These and other objects, features and advantages of the present invention will be more clearly understood through a consideration of the following detailed description.